A wide variety of research has been conducted in the field of biocompatible polymers. The definition of the term “biocompatible” depends on the particular application for which the polymer is designed. In the field of ophthalmic lenses, and in particular in the field of contact lenses, a biocompatible lens is generally defined as one that does not substantially damage the surrounding ocular tissue and ocular fluid during the time period of contact. The phrase “ophthalmically compatible” more appropriately describes the biocompatibility requirements of ophthalmic lenses.
One ophthalmic compatibility feature for contact lenses is that the lens permits oxygen to reach the cornea in an amount that is sufficient for long-term corneal health. The contact lens should permit oxygen from the surrounding air to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissues. If sufficient oxygen does not reach the cornea, corneal swelling can occur, as can epithelial microcysts, stromal/epithelial thinning, stromal acidosis, endothelial polymegethism, corneal ulcers and increased inflammation. Brennan et al., Opt. Vis. Sci., 74(8):609–623 (1997). “Soft” contact lenses conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Thus, soft contact lenses should permit oxygen to diffuse through the lens to reach the cornea.
Another ophthalmic compatibility feature for soft contact lenses is that the lens not adhere strongly to the eye. Clearly, the consumer should be able to easily remove the lens from the eye for disinfecting, cleaning or disposal. Moreover, the lens should also be able to move on the eye in order to encourage tear flow between the lens and the eye. Tear flow between the lens and eye permits debris, such as foreign particulates or dead epithelial cells, to be swept from beneath the lens and, ultimately, out of the eye in tear fluid. Thus, a contact lens should not adhere to the eye so strongly that adequate movement of the lens on the eye is inhibited.
Although there exist rigid gas permeable (“RGP”) contact lenses that have high oxygen permeability and that move on the eye, RGP lenses are typically quite uncomfortable for the consumer. Many consumers therefore prefer soft contact lenses. Moreover, a contact lens that can be continuously worn for a period of a day or more (including wear during periods of sleeping) implies comfort levels that exclude RGP lenses as popular extended-wear candidates.
In order to balance the ophthalmic compatibility and consumer comfort requirements in designing a daily wear soft contact lens, polymers and copolymers of 2-hydroxyethyl methacrylate (HEMA), N-vinyl pyrrolidone and glyceryl methacrylate (GMA) were developed. These hydrophilic polymers move well on the eye and provide sufficient oxygen permeability for daily wear (e.g. about 8–35 Dk units). The FDA has approved certain soft contact lenses for extended wear periods of up to about six nights of overnight wear and seven days of daily wear. However, the consumer cannot safely and comfortably wear these poly(HEMA) lenses for extended periods of seven days or more, because the oxygen permeability is insufficient. True extended wear (i.e., seven days or more) of these lenses can result, at a minimum, in corneal swelling and development of surface blood vessels in the cornea.
In order to improve oxygen permeability, polymers containing silicone groups were developed. Many siloxane-containing polymers have been disclosed as having high oxygen permeability. [For example, see U.S. Pat. Nos. 3,228,741; 3,341,490; 3,996,187; and 3,996,189.] However, contact lenses made using those known polysiloxanes often adhere to the eye, inhibiting the necessary lens movement. Polysiloxanes are typically highly lipophilic, which causes a haze of lipids and proteins to form that interferes with vision through the lens.
There have been attempts to blend the desirable properties of hydrophilic polymers, formed from monomers such as HEMA, with the desirable oxygen permeability of polymers formed from siloxane-containing monomers. [For example, see U.S. Pat. Nos. 3,808,178; 4,136,250; 4,711,943; 5,070,169 and 5,760,100.] However, prior attempts at producing a true extended-wear contact lens have not been particularly successful, either because of the effect of the extended-wear lens on corneal health or because the lens would not move on the eye. Thus, oxygen permeabilities remained too low and/or the lenses adhered to the corneas.
Thus, there remains a need for an ophthalmically compatible, transparent polymeric material that is suited to prolonged periods of continuous contact with ocular tissue and tear fluid. The discussion that follows discloses such materials, contact lenses made from those materials and processes for preparing the lenses.